cgroup: reorder SUBSYS(blkio) in cgroup_subsys.h

The scheduled cgroup writeback support requires blkio to be
initialized before memcg as memcg needs to provide certain blkcg
related functionalities.  Relocate blkio so that it's right above
memory.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup_subsys.h

@@ -15,6 +15,10 @@ SUBSYS(cpu)
 SUBSYS(cpuacct)
 #endif
 
+#if IS_ENABLED(CONFIG_BLK_CGROUP)
+SUBSYS(blkio)
+#endif
+
 #if IS_ENABLED(CONFIG_MEMCG)
 SUBSYS(memory)
 #endif
@@ -31,10 +35,6 @@ SUBSYS(freezer)
 SUBSYS(net_cls)
 #endif
 
-#if IS_ENABLED(CONFIG_BLK_CGROUP)
-SUBSYS(blkio)
-#endif
-
 #if IS_ENABLED(CONFIG_CGROUP_PERF)
 SUBSYS(perf_event)
 #endif